When a certain type of phosphor is irradiated with radiation such as X-rays, α-rays, β-rays, γ-rays, electron rays, or UV rays, part of the radiation energy is accumulated in the phosphor. When this phosphor is irradiated with excitation light such as visible light, the phosphor exhibits stimulated luminescence corresponding to the accumulated energy. A phosphor having such a characteristic is called a cumulative phosphor or stimulated phosphor.
Conventionally, a radiographic image is obtained as an electrical image signal by using a cumulative phosphor. More specifically, radiographic image information of an object such as a human body is temporarily recorded on a cumulative phosphor sheet. The cumulative phosphor sheet is scanned by using excitation light such as a laser beam to generate stimulated luminescent light. The obtained stimulated luminescent light is photoelectrically read to obtain an image signal. A radiographic image information recording/reproduction system which causes a recording material such as a photosensitive material or a display apparatus such as a CRT to output the radiographic image of an object as a visible image on the basis of the image signal is proposed in, e.g., Japanese Patent Laid-Open No. 55-12429 or 56-11395. This method is called computed radiography (CR).
In recent years, a system which obtains the radiographic image of an object by using a semiconductor image sensor having a large area (flat panel detector: FPD) has been developed. This system which can record an image in a very wide radiation exposure area is more advantageous for practical use than a conventional radiographic system that uses a silver halide photo. The procedures of image acquisition in a system of this type are as follows. Dynamic range X-rays in a very wide area are read as an electrical signal by using a photoelectric conversion means. The electrical signal is further converted into a digital signal. The digital signal is processed and output to a recording material such as a photosensitive material or a display apparatus such as a CRT as a visible image. Accordingly, a satisfactory radiographic image can be obtained even when the radiation dose varies to some extent.
Even in mammography, imaging can be executed by using a flat panel detector (FPD) having a pixel size of 50 to 100 μm. In mammography, a compression plate which compresses a breast is used to uniform X-rays that pass through the breast. Various developments are progressing for the compression plate.
Japanese Patent Laid-Open No. 3-86154 discloses a mammographic apparatus having a compression mechanism for manipulating a compression plate that compresses a breast inserted between the compression plate and an imaging table on the X-ray irradiation beam path. The compression mechanism can freely move the compression plate back and forth and/or to the left or right with respect to the subject on a spatial plane that is perpendicular to the X-ray irradiation beam path. The mechanism can also fix and hold the compression plate at a specific moved position.
Japanese Patent Laid-Open No. 2-504353 discloses a mammographic apparatus whose breast holder is constituted by a lower holder and an upper holder. The lower and upper holders can freely displace from each other so that a breast to be imaged is compressed between the lower holder and the upper holder. The frame portion of this apparatus is rotated about or located near an axial line that almost coincides with the central axial line of the breast compressed by the breast holder. Hence, when photo projection or the mode is to be changed, the patient to be imaged need not move or the height position of the apparatus need not be changed.
Japanese Patent Laid-Open No. 10-234716 discloses an X-ray apparatus in which the X-ray generating section and X-ray receiving section are fixed at two ends of a support member. A table is inserted between the X-ray generating section and the X-ray receiving section. The X-ray apparatus has a compression bar driving section which drives a compression bar to compress, e.g., the abdominal part of a patient on the table. The compression bar driving section is supported to be rotatable independently of the rotational direction of the support member. This apparatus executes X-ray imaging of a abdominal part. The X-ray generating section and X-ray receiving section can rotate independently of the compression bar for the abdominal part.
In mammography, a tumor and calcification must be detected. Conventionally, imaging is executed only in two, CC (CranioCaudal) and MLO (MedioLateral Oblique) directions. Hence, a morbid part distribution can hardly be recognized as a stereoscopic vision. If the three-dimensional structure of a tumor and the distribution of a plurality of calcified parts can be recognized in an obtained image, it is expected to be useful for distinction between benign and malignant tumors.
In Japanese Patent Laid-Open No. 3-86154, the compression plate can be moved to suitably align the object and compression plate at the time of X-ray imaging. However, since X-ray imaging is executed in a fixed state, no X-ray image that allows stereoscopic observation can be obtained. In Japanese Patent Laid-Open No. 10-234716, the X-ray generating unit and X-ray receiving section can rotate about the compression bar. However, this apparatus aims at X-ray imaging of a abdominal part. This apparatus is therefore inappropriate for imaging of an object that is deformed by a compression plate as in mammography.
In X-ray imaging such as stereography or rotational DSA (angiography), the three-dimensional distribution of morbid parts in a human body is observed by changing the incident angle of X-rays that become incident on the object. In mammography, however, the motion of the apparatus is restricted because of the relative positional relationship between the apparatus and the human body. For this reason, it is difficult to obtain an image that allows stereoscopic observation by applying the above methods to a mammographic apparatus as in Japanese Patent Laid-Open No. 2-504353. There may also be an idea of applying tomography to mammography. However, the idea is not good because, e.g., the apparatus becomes bulky.
Under the circumstances of the prior arts described above, a radiographic apparatus and method which can execute stereoscopic imaging of an object or acquire three-dimensional information with a simple arrangement are demanded.